Contents lists available at ScienceDirect Estuarine, Coastal and Shelf Science journal homepage: www.elsevier.com/locate/ecss Iron cycling in a mesocosm experiment in a north Patagonian fjord: Potential effect of ammonium addition by salmon aquaculture N. Sanchez a,* , N. Bizsel b , J.L. Iriarte c , L.M. Olsen d , M.V. Ardelan a a Norwegian University of Science and Technology (NTNU), Department of Chemistry, Trondheim, 7491, Norway b Institute of Marine Sciences and Technology, Dokuz Eylul University, H. Aliyev Bulv., No: 10, Inciralti, Izmir, Turkey c Instituto de Acuicultura and Centro Dinámica de Ecosistemas Marinos de Altas Latitudes - IDEAL, Universidad Austral de Chile, Puerto Montt. COPAS Sur-Austral, Universidad de Concepción, Concepción, Chile d Norwegian Polar Institute, Fram Centre, 9296, Tromsø, Norway ARTICLE INFO Keywords: Iron Ammonium Phytoplankton assemblage structure Particle export Fjords Salmon aquaculture ABSTRACT Salmon aquaculture in Chile has been a rapidly growing industry, generating increasing inputs of organic matter and inorganic nutrients into the ecosystem. We studied the potential impacts of ammonium input by this in- dustry on the cycling of iron (Fe) in a Chilean fjord. The distribution of different Fe fractions at varying am- monium concentrations was monitored in a twenty-two day mesocosm experiment. The setup involved brackish water and seawater; each had a control and four ammonium concentrations. Measurements were performed for total (TFe Ch ) and dissolved (DFe Ch ) chelex labile Fe fractions, and particulate Fe (PFe). Results for both brackish and seawater showed similar trends but differences in magnitude. Over time, DFe Ch decreased with increasing ammonium concentration, while TFe Ch showed up to a three-fold increase positively correlated with ammonium addition, chlorophyll and particulate organic carbon. Overall, PFe values increased over time with 37%–89% of this fraction estimated to be of lithogenic origin. When normalized to particulate organic carbon and chlor- ophyll, PFe was negatively correlated with ammonium showing an exponential decrease. The PFe measured in the 20–140 μm fraction, showed a hyperbolic relationship with particulate phosphorus, suggesting a change in the ratio for these elements in this size fraction. The increase and dominance of diatoms over time in both water types, together with the observed PFe trend, suggest that large phytoplankton potentially act as the main carrier phase of potential scavenged Fe via the available surfaces of sinking cells. Positive correlations between changes in TFe Ch and changes in chlorophyll and particulate organic carbon suggest a biological role in controlling the particulate labile Fe fraction, hence resulting in a potential increase of bioavailable Fe. Increasing ammonium addition in the fjords of Chile caused by salmon aquaculture may affect the phytoplankton assemblage com- position and therefore the PFe to organic carbon ratio. Possible changes in biogeochemical Fe cycling may result from nutrient enhanced diatom-dominated blooms acting as more efficient vectors for downward export of organic matter. 1. Introduction The fjord ecosystem in Chile constitutes a nearly pristine environ- ment. However, it is experiencing growing anthropogenic influences, mainly from aquaculture. The aquaculture industry has increased in the last two decades, causing growing concerns about its impact on the environment. Salmon aquaculture releases nutrients to the water column mainly as dissolved inorganic components (ammonium and phosphate) through excretion and particulate organic components (particulate nitrogen and phosphorus) from fish feces (Olsen et al., 2008). Oxygen depletion and decreased biodiversity are known effects of aquaculture on marine sediments and benthic fauna. Although im- pacts on benthic ecosystems from aquaculture activities are well documented (Buschmann et al., 2006; Strain and Hargrave, 2005; Soto and Norambuena, 2004), knowledge about how extra nutrient input by aquaculture affects the structure and functions of the pelagic ecosystem is still scarce (Cloern, 2001; Olsen et al., 2006, 2008). Theses anthro- pogenic inputs have the potential to alter the nutrient stoichiometry in seawater and therefore, to some extent, affect the biogeochemical cy- cling of major elements on a regional basis (Arrigo, 2005). In the marine environment major biogeochemical cycles such as carbon, nitrogen and phosphorus strongly depend on marine https://doi.org/10.1016/j.ecss.2019.02.044 Received 10 December 2017; Received in revised form 7 January 2019; Accepted 20 February 2019 * Corresponding author. E-mail address: nicolas.sanchez@ntnu.no (N. Sanchez). Estuarine, Coastal and Shelf Science 220 (2019) 209–219 Available online 23 February 2019 0272-7714/ © 2019 Published by Elsevier Ltd. T